The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.
The present invention relates generally to analytical methods for measuring physical properties of materials, and more particularly to a novel method for measuring strain in brittle foams and for calculating Poisson""s ratio in the foams.
The superior high temperature properties, specific stiffness and ultra-light weight make open cell carbon foam an attractive multifunctional structural material for many space, airframe and commercial applications, such as thermal planes in space structures, core materials in aerospace sandwich structures, isotropically reinforced precursors for net shape fabrication, a reinforcing medium for new generation aircraft brakes, and a core material in the commercial recreation market. The ultra-light weight of the material is achieved through high porosity (80% or greater) in the material containing a tetrahedral cell ligament microstructure. Successful structural application of the material requires accurate knowledge of the mechanical properties of the material. Poisson""s ratio is one property that is required for complete and accurate structural design assessment.
Poisson""s ratio in a material is defined as the ratio of lateral strain to longitudinal strain as measured in two orthogonal directions. In the measurement of Poisson""s ratio in brittle foams, conventional strain measuring devices, such as extensometers and strain gages, are not satisfactory because the porous foam surface allows ingress of any adhesive used to mount these devices, and the inherent stiffness of the devices causes a stiffening effect that results in false measurements of true strain in the material. Alternative non-contact measurement techniques for measuring strain in materials generally include laser interferometry, speckle interferometry and holography, all of which are instrument sensitive and require time consuming and elaborate experimental setup. None of the prior art methods just described are entirely satisfactory for measuring Poisson""s ratio in foams, and no previously existing non-contact strain measuring method provides simultaneous measurement of strains in porous materials in multiple directions.
The invention solves or substantially reduces in critical importance problems with conventional methods for measuring shear stiffness of isotropic foam materials by providing an accurate, low cost, non-contact strain measuring method and apparatus for measuring Poisson""s ratio of porous, low modulus (brittle) materials, including foams such as carbon foam, metallic foam, polymeric foam, ceramic foam, reticulated vitreous carbon, and fuel cell materials, using optical microscopy. According to the invention, strains in multiple directions in the material may be measured simultaneously without physical contact with the surface of the material, which ensures accuracy and reliability of strain measurement in porous materials.
In accordance with a preferred embodiment of the invention, one or more reference marks, such as in the form of grid lines in the orthogonal x and y directions, are applied to a surface of a specimen using fluorescent ink, and the specimen is mounted on the stage of an optical microscope to which an x-y digital micrometer is attached. The surface is viewed through the microscope using a source of light source that renders the lines visible, and selected reference points on the grid lines are identified for strain monitoring. A specially designed load frame is used to apply a load to the specimen by inflating a diaphragm that exerts a tensile load on the specimen. The crosshair of the microscope objective lens is aligned with the selected reference point(s) so that changes in displacement for each incremental loading may be recorded using the digital micrometer.
It is therefore a principal object of the invention to provide a method for measuring strains in multiple directions in porous, low modulus (foam) materials.
It is another principal object of the invention to provide a method for measuring Poisson""s ratio in low modulus materials.
It is another object of the invention to provide a method and apparatus for determining shear stiffness in isotropic foam materials associated with measuring Young""s modulus.
It is another object of the invention to provide a novel method for measuring Poisson""s ratio in brittle foam materials.
It is yet another object of the invention to provide a simple and inexpensive method and apparatus for non-contact measurement of strain in porous materials.
It is yet another object of the invention to provide method and apparatus for non-contact simultaneous measurement of strain in porous materials in multiple directions.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.
In accordance with the foregoing principles and objects of the invention, an accurate, low-cost, non-contact strain measuring method and apparatus for measuring strain in low modulus porous materials using optical microscopy are described wherein, in a preferred embodiment of the invention, fluorescent ink reference lines are applied to a surface of a specimen, the specimen is mounted on the stage of an optical microscope having an attached x-y digital micrometer, and a load is applied to the specimen. The surface is viewed using a light source that renders the reference lines visible, and changes in displacement of selected points on the reference lines are measured for each incremental loading. Poisson""s ratio may be calculated from the strain measurements in two orthogonal directions.